Arc discharge lamps are known to be both efficient and to produce a high quality white light. In essence, an electric arc is formed by an electric potential between two electrodes causing electrons to flow between the electrodes thereby heating the intermediate gas. The heated gas discharges by radiating light and is three to four times more efficient than a heated tungsten filament at producing light. The arc discharge may also have a higher brightness than tungsten. Historically arc discharge lamps were formed with openly exposed electrodes. These early lamps were large, consumed large amounts of electricity, were very hot but had a large light output. The early arc discharge lamps were used as search lights and movie stage lights. Currently small, enclosed arc discharge lamps are made for many special applications, and the trend generally is to reduce the size, power, and heat of a discharge lamp. The high efficiency and high quality light of an arc discharge lamp can then be brought to the small scale of common lighting needs.
To separate the electrodes and help prevent arc over between the inputs, arc discharge lamps are frequently double ended. The two lamp leads then approach the lamp from opposite sides thereby enhancing the separation between the leads. To increase the efficient light output of the lamp, a reflector may be added which commonly supports a portion of the lamp. The double ended lamp may be oriented transverse to the reflector axis, but since the arc is approximately linear between the electrodes, it is usually more useful to orient the electrodes so the arc is parallel with the reflector axis. One lamp lead is then brought through the reflector base near the point where the optical axis meets the reflector, while the other second lead is connected straight out the front of the lamp. The front connection may be made by an appropriately convenient means. Examples of the open ended arc discharge reflector lamps are shown in U.S. Pat. Nos. 3,624,600; 3,700,881 and 3,988,626.
Arc lamps like other lamps are subject to dirt, weather, probing human hands and other interferences. Covering the reflector with a lens protects the lamp structure, and preserves the reflector surface. The problem arises as to how the front electrode may be connected. One solution is to extend the front electrode connection through the lens cover, and make whatever convenient connection is available in front of the lens. Examples of sealed arc discharge reflector lamps where the electrode extends through the lens may be seen in U.S. Pat. Nos. 3,364,374; 3,684,908, 4,290,097, 4,423,348 and 4,686,419.
An electrode extending from the front of a lamp invites electrical and mechanical problems. When arc discharge lamps were special lights, the exposed elements could be expected to be treated with care by attentive users. As discharge lamps become more common, less care can be anticipated, so a means of enclosing the front lead has developed. The front of the lamp is connected inside the enclosed reflector. The front lead is then ducted out through the reflector body at a position offset from the rear electrode. The difficulty is arc discharge lamps rely on a high voltage difference for restart, and reflectors are commonly metal, or metalized surfaces. The reflector can then act as a pathway for arc over between the lamp leads. Examples of sealed arc discharge reflector lamps where the electrode is connected inside an enclosed reflector may be seen in U.S. Pat. Nos. 3,610,912, 3,731,133, 3,808,496, 4,423,471 and 4,724,352.
Since an arc discharge lamp has higher brightness, and proportionately less infrared radiation, automobile headlamps may be made that are more efficient, and more compact. Tungsten filament lamps commonly have a rated lifetime of about 500 to 1000 hours, but arc discharge lamps typically have a lifetime of thousands of hours. Filaments may break under the jouncing stresses of road conditions, but an arc discharge lamp has no filament. An automobile arc discharge lamp can then be designed to last for the life of a car.
In arc discharge lamps, unlike tungsten filament lamps, the range of useful light output may be significantly altered by changing the applied current, surrounding magnetic or electric fields. Arc discharge lamps may output light with a color temperature from 2000 to 10,000 degrees Kelvin, while tungsten lamps may range from 2000 to 3000 degrees Kelvin. Discharge lamps can also operate with currents that vary by a factor of two or more, while tungsten lamps have a severely degraded lifetime if operated at only a few percent above specification. In combination then, discharge lamps offer the possibility of being operated to change direction, intensity or color according to the specific weather, and road conditions. Low beams can be turned to be high beams, and regular lights can become fog lights. An arc discharge automobile headlamp offers the possibility of a better, more flexible lamp, in a smaller package that rarely if ever needs to be replaced.
Automobile headlights are particularly demanding on the lamp specifications. Weather, dirt, constant jarring, and other environmental affects have to be taken into consideration in a practical design. Specific light patterns, light quality, and requirements for durability and long life are set by regulatory agencies, and car manufacturers. The focus and beam pattern of an automobile headlamp is critical to its legal and commercial acceptance. With small volume headlamps the focal point is very finely defined. In general, the smaller the lamp package, the more exacting the positioning of the light source. Tungsten lamps are usually positioned mechanically with the aid of feed back from positioned flux sensors. Arc discharge headlights are now being adapted for use as automobile headlamps. Examples of include U.S. Pat. No. 4,594,529 issued to Bertus de Vrijer on Jun. 10, 1986 for a metal halide discharge lamp; U.S. Pat. No. 4,722,039 issued Jan. 26, 1988 to Manfred Gaugel for a shaded beam vehicular discharge type head lamp, and Japanese Pat. No. 62-131459 disclosed Jun. 13, 1987 for capped discharge lamps for headlights. Applicants show in two related U.S. patent applications, Ser. Nos. 144,828 and 144,836 both filed on Jan. 14, 1988, a magnetic means to control an automobile headlight with an arc discharge lamp positioned in a sealed reflector cavity.
To produce instant light, that is, light immediately after switching the light on, an arc discharge lamp capsule requires high gas pressure, which inturn requires a high ignition voltage. When the lamp is cold, the lamp fill pressure may be less than one atmosphere, in which case the minimal cold starting voltage may be as small as 2000 volts, depending on the electrode separation, starting voltage pattern, and other known lamp aspects. After the lamp has been on, the internal lamp pressure may rise because of fill temperature to ten, twenty, thirty or more atmospheres. Since, the ability to draw electrons across the electrode gap is approximately proportional to the product of the pressure and the gap distance, hot restart requires a voltage that is then ten, twenty, thirty or more times as great as the minimal cold start voltage.
A reliable instant hot start is clearly necessary in an automobile headlamp, and high or very high voltages are therefore considered necessary. High voltages are progressively more difficult to insulate. Vacuum and gas are effective insulators. Any close positioning of input leads invites arc over, resulting in a failure to restart the lamp. For example, high voltage arcing may occur along the envelope surface between the lead entry points in a capsule, across open spaces to nearby objects, or through wet, oily, aged or otherwise degraded insulation. Lead separation and high quality lead insulation are important to assure hot restart.
As a comparison, automobile spark plug wires commonly conduct voltages of about 20,000 volts. It is common knowledge that spark plug wires age, and leak to nearby objects, especially when wet, soiled or oily. The high voltages might also arc to nearby car parts, or humans if the electrode was ducted through the front of the lamp or the side of the reflector. Similarly, to reduce headlight volume, the reflector cavity must be small, which therefore limits the possible lamp lead separation in the reflector cavity.
Including the lamp leads inside the lamp capsule is not felt to adequately insulate the leads. Arcing can occur along glass surfaces, or across lamp cavities between metal sections. High electric fields induced by high voltages can devitrify glass, and otherwise break down insulating materials in time. Also, even if the arc over is prevented inside the lamp capsule, the same problems can arise between the leads in places exterior to the capsule.
More broadly, to gain greater performance from headlights while at the same time reducing the size of the lamp enclosure, the methods of producing light are generally pushed to use more extreme conditions. The extreme conditions in turn at times need special controls, and protections; and inturn the users of sophisticated headlights may need protection from the conditions, and controls. There is then a need to provide a headlamp with sophisticated controls without being degraded by the adverse conditions in an automobile, and without offering possible harm to ordinary users.
Various arc discharge headlamps systems are shown by existing patents or published patent applications. EPO 231,936 to Fritz Eckhardt, DG 245,080; to Uwe J. Among et al; EPO 245,735 to Jurgen Heider et al.; DG 3519627 to Manfred Gaugel et al.; DG 3040583 to Jurgen Fischer; EPO (application) to Shinji Inuki et al.; UK 2,186,957 to Toru Segoshi; EPO (application) 0,224,954 to Rudolf Sanders; UK (application) 2,123,541 to Gyorgy Szekacs.